Field of the Invention
The present invention relates to semiconductor devices, and, in particular, to the fabrication method of semiconductor devices having lightly doped drains.
Description of the Related Art
In the processes used for manufacturing semiconductor devices, the sizes of these devices are continuously being reduced, largely by increasing the density of the devices and thereby enhancing their operation speed and performance. Take the MOS transistor for example: decreasing the length of the channel and the thickness of the gate oxide induces an increase of the vertical and horizontal electric field. The increased horizontal electric field accelerates the speed of carriers in the channel. When these carriers, which have been accelerated in the channel, have enough energy, this increases the probability that these carriers emit in, over, and through gate oxide to form a gate current, or to induce electron-hole pairs at the drain region. Therefore, when the channel of an MOS transistor shrinks, the amount of carriers in the channel near the drain region is increased to induce the hot electron effect. In the hot electron effect, some of the electrons are emitted in gate oxide, and the corresponding holes flow into the substrate and produce the substrate current. Some of the other holes are collected by the source region, and the effect of the NPN-bipolar transistor is enhanced, multiplying more carriers, and even causing an electrical breakdown.
With the above issues, conventional techniques provide lightly doped drains (LDD) to reduce or prevent semiconductor devices from being damaged by the hot electron effect. Using traditional fabricating methods, the lightly doped drain still needs to be improved, and the purpose of the present invention is to provide new fabricating methods of semiconductor devices having lightly doped drains.